1. Field of the Invention
The present invention relates to a scanning apparatus which may be used in a real-time imaging system, and in particular in a real-time mm wave length, or microwave imaging system.
2. Summary of the Prior Art
In optics, there is a requirement to scan beams rapidly across the scene in a linear scan pattern.
For visible light, it is known to achieve a linear scan pattern by the use of two circular rotating reflective discs, which are equally inclined to their axes of rotation, and which rotate at the same speed. A single rotating mirror would produce a conical scan. However, when the second rotating mirror receives a conical scan from the first rotating mirror parts of the conical scan are effectively cancelled out resulting in a linear scan pattern. Alternatively flapping mirrors may be used to achieve a linear scan pattern.
Such methods are suitable in the visible region where the apertures involved are small. In the infra-red region apertures are frequently about 100 mm and focussing lenses are used to reduce this aperture to around 10 mm. In the mm-wave and microwave regions these apertures are very large (in the order of 1 m) and it is not practical to use flapping mirrors. Also when making reflective discs, there is considerable displacement of the beam which is received by the second of the mirrors, and therefore the second mirror has to be very large so as to be able to receive the displaced beam.
It is also known, for visible light, to produce a linear scan using two counter rotating prisms. However, such prisms are heavy and therefore not suitable for large aperture systems.